A normal volume of amniotic fluid (AF) is essential for normal fetal development with favorable perinatal outcome. However the mechanisms that regulate AF volume and the factors that maintain the volume within the physiological range are not well understood. The current understanding suggests that the transfer of AF water and solutes across the fetal membranes into fetal blood vessels that vascularize the surface of the placenta is the pathway where regulation occurs. This intramembranous (IM) pathway for AF absorption is constituted by an active bulk transport component and a passive diffusional component. In addition, the active process is regulated by stimulatory and inhibitory factors in the AF and amniotic membrane. Although the existence of these regulatory factors has been proposed, their identity and mechanisms of action are not known. In this application, we propose to elucidate the cellular pathways of IM transport and decipher the factors that regulate these pathways. These studies will be carried out in ovine amnion cells in vitro and chronically catheterized ovine fetuses in vivo. In Specific Aim 1, we will identify the cellular pathway for transport of solutes across amnion cells and test the hypothesis that AF transport is a vesicular transcytotic process via caveolae. We will investigate the role of VEGF165 as a stimulator and VEGF165b as an inhibitor of caveolar transport, as well as the effect of the soluble VEGF receptor 1 (sVEGFR-1) in antagonizing VEGF bioactivity. Specific Aim 2 will determine VEGF165 and VEGF165b mRNA and protein levels in amnion cells and amniotic membranes under conditions of normal, increased or decreased IM absorption rates. The correlation of VEGF165 levels with sVEGFR-1 will be determined. In Specific Aim 3, we will examine the VEGF165 activation of caveolar transcytosis by induction of VEGF receptor 2 to initiate a c-Src signaling pathway leading to downstream activation of cavoelin-1 and dynamin-2 as required for caveolar endocytosis and transcytosis. The involvement of other signaling proteins including protein kinase C and phosphatidylinositol 3-kinase will be explored. Specific Aim 4 will investigate the expression of the water channel proteins aquaporin 1, 3 and 9 in amnion cells and determine their effects on passive and active transport across amnion cells. In Specific Aim 5, we will evaluate the in vivo function of the stimulator VEGF165 and the inhibitors VEGF165b and sVEGFR-1 in modulating IM absorption rate in ovine fetuses under conditions of normal, increased or decreased AF volume. We anticipate the in vivo results to support the in vitro findings that VEGF165 is an important determinant of IM absorption and that its stimulatory effect is antagonized by VEGF165b and sVEGFR-1. Overall these studies will elucidate the transcellular vesicular pathway for AF transport and determine the stimulatory and inhibitory regulatory factors that modulate this pathway in amnion cells. Further, the signal transduction cascades that mediate these transport events will be investigated. The findings will lead to an improved understanding of the etiology of amniotic fluid volume abnormalities.